1. Field of the Invention
The invention relates to an illuminating device, more particularly to a direct type illuminating device.
2. Description of the Related Art
Backlight modules are used as illuminating light source devices for liquid crystal displays. Therefore, distribution of the illumination intensity of the backlight module needs to be uniform for the liquid crystal display to have a fine image quality.
As shown in FIG. 1, a conventional direct type backlight module 11 includes a reflector plate 111, a diffuser plate 112 and a plurality of light source units 113 arranged at intervals between the reflector plate 111 and the diffuser plate 112. The light source units 113 provide source light beams that illuminate toward the reflector plate 111 and the diffuser plate 112. The reflector plate 111 assists in directing most of the source light beams toward the diffuser plate 112. Generally, the diffuser plate 112 is made by injection molding a highly transmissive material blended with microscopic scattering particles. The highly transmissive material can be polycarbonate (PC) or polyethylene terephthalate (PET). The source light beams are diffused by the scattering particles and are uniformed when passed through the diffuser plate 112.
In order to make sure that the illumination intensities of the source light beams provided by the light source units 113 on the diffuser plate 112 are uniformly distributed (as illustrated by curve (A) in FIG. 2), cooperation between the distance (H) between the diffuser plate 112 and the reflector plate 111 and the distance (L) between adjacent light source units 113 is required. For the purpose of saving space, the distance (H) between the diffuser plate 112 and the reflector plate 111 needs to be kept small. If the distance (L) between adjacent light source units 113 is kept constant, while the distance (H) between the diffuser plate 112 and the reflector plate 111 is reduced, then the illumination intensity on the diffuser plate 112 would be unevenly distributed (as illustrated by curve (B) in FIG. 2).
Therefore, when the distance (H) between the diffuser plate 112 and the reflector plate 111 is reduced, the distance (L) between adjacent light source units 113 needs to be decreased as well such that the Gaussian-shaped illumination intensity distributions of the source light beams provided by adjacent light source units 113 on the diffuser plate 112 have enough overlapped areas so as to ensure an overall uniform distribution of illumination intensity on the diffuser plate 112. However, decrease of the distance (L) between adjacent light source units 113 also means an increase in the number of light source units 113 needed in the conventional direct type backlight module 11, which results in an increase in the number of passive elements, such as direct current alternating current inverters (DC-AC inverters) (not shown), necessary for operation of the light source units 113. Consequently, the fabrication cost of the conventional direct type backlight module 11 is increased.
Several conventional methods for improving the distribution of illumination intensity while reducing the thickness of the backlight module (i.e., the distance between the diffuser plate and the reflector plate) are provided hereinbelow. Referring to FIG. 3, U.S. Patent Publication No. 2005/0248939 A1 discloses a lens-arrayed backlight module 13, which utilizes a plurality of lens plates 133 to change the path direction of source light beams provided by light source units 132 and traveling away from housing plates 131. As a result, the Gaussian-shaped illumination intensity distribution of the source light beams provided by individual light source units 132 and traveling away from the housing plates 131 is relatively smooth, and consequently, the illumination intensity distribution of the lens-arrayed backlight module 13 is kept uniform while the thickness thereof is reduced. Referring to FIG. 4, U.S. Patent Publication No. 2006/0133092 A1 discloses a backlight module 14 including a plurality of light diffusing devices 146, each of which is disposed above a respective light source unit 145 so as to change path direction of source light beams provided by the light source units 145 and traveling away from a reflector plate 142 such that the illumination intensity distribution on a top plate 143 is uniform. However, since the lens plates 133 or the light diffusing devices 146 of the abovementioned backlight modules 13, 14 are disposed only at one side of the light source units 132, 145, the source light beams traveling toward the housing 131 or the reflector plate 142 are still not modulated. As a result, the effect of providing a uniform illumination intensity distribution is limited, and the reduction in thickness of the backlight modules 13, 14 is also limited.
Further, referring to FIG. 5, Taiwanese Patent No. 200877 discloses an illuminating device 15 utilizing light bulbs 154, each of which is confined in a cavity of a respective light diffuser 153 such that light beams provided by the light bulbs 154 are scattered and path directions thereof are changed. However, this method only provides that the light beams are scattered randomly, and is therefore not a systematic way of modulating the light beams. Consequently, control over the illumination intensity distribution of the illuminating device 15 is relatively difficult.